1-Azaniumylcyclobutane-1-carboxylate monohydrate

In the title compound, C5H9NO2·H2O, the amino acid is in the usual zwitterionic form involving the α-carboxylate group. The cyclobutane backbone of the amino acid is disordered over two conformations, with occupancies of 0.882 (7) and 0.118 (7). In the crystal, N—H⋯O and O—H⋯O hydrogen bonds link the zwitterions [with the water molecule involved as both acceptor (with the NH3 +) and donor (through a single carboxylate O from two different aminocyclobutane carboxylate moities)], resulting in a two-dimensional layered structure lying parallel to (100).

In the title compound, C 5 H 9 NO 2 ÁH 2 O, the amino acid is in the usual zwitterionic form involving the -carboxylate group. The cyclobutane backbone of the amino acid is disordered over two conformations, with occupancies of 0.882 (7) and 0.118 (7). In the crystal, N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds link the zwitterions [with the water molecule involved as both acceptor (with the NH 3 + ) and donor (through a single carboxylate O from two different aminocyclobutane carboxylate moities)], resulting in a two-dimensional layered structure lying parallel to (100).
RJB wishes to acknowledge the NSF-MRI program (grant CHE-0619278) for funds to purchase the diffractometer. GB wishes to acknowledge support of this work from NASA (NNX10AK71A)

Comment
The alpha amino acids are essential for life as they are the building blocks of all proteins and enzymes. Nature uses almost exclusively the L form of the nineteen common chiral amino acids. The title compound is achiral due to the symmetry of the alpha carbon atom. It also lacks a hydrogen atom on the alpha carbon atom, which is critical in the racemization of the proteinogenic amino acids (Yamada et al., 1983). Little is known about the mechanism of racemization of amino acids lacking an alpha hydrogen atom (Pizzarello & Groy, 2011). Mechanistic investigation of racemization pathways of appropriate derivatives of the title compound may shed light on the racemization of this class of amino acids. Over eighty amino acids that have been identified in meteorites (Burton et al., 2012;Pizzarello et al., 2006). The title compound has not been detected to date in extraterrestrial sources but a higher analog, cycloleucine, has been reported (Pizzarello et al., 2004). The title compound has been incorporated into peptides for conformational studies on model proteins with 1-aminocyclobutane-1-carboxylic acid residues (Balaji et al., 1995). In addition the title compound has been shown to inhibit the enzyme 1-aminocyclopropane-1-carboxylate synthase, part of the pathway for ethylene production that leads to the ripening and spoilage of fruit (Nakatsuka et al., 1998;Bulantseva et al., 2003). The structures of some related non-proteinogenic amino acids have recently been reported .
The structure of the title compound has not been reported to the CCDC but there is a report of its hydrochloride salt as a monohydrate (Chacko & Zand, 1975). In the structure of the title compound the amino acid is in the usual zwitterionic form involving the α carboxylate group and all the the bond lengths and angles are in the normal range for such compounds (Orpen, 1993). The metrical parameters of the title compound and its hydrochloride salt do not differ significantly apart from the C-O bond lengths which are equivalent in the title compound but differ significantly in the hydrochloride salt. The cyclobutane backbone of the amino-acid is disordered over two conformations with occupancies of 0.882 (7) and 0.118 (7). There is extensive N-H···O and O-H···O hydrogen bonding (Table 1) linking the zwitterions into a two-dimensional layered structure lying parallel to (100) (Fig. 2).

Experimental
1-Aminocyclobutane carboxylic acid was purchased from Sigma Aldrich. Crystals of the title compound were grown by evaporation from an aqueous solution of the achiral amino acid.

Special details
Experimental. Absorption correction: CrysAlisPro (Agilent, 2012) Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (